The planets all have gravity, which pull all the other planets just a little. Over a very long timescale this results in most of the planets lying in the same plane, like concentric circles on a piece of paper, and orbiting in the same direction. They are not exactly flat, they differ by a couple of degrees, but it's pretty flat. A couple of stray rocks and comets are much more inclined, some even perpendicular, but the planets themselves are very flattish, and this is known as the Plane of the Ecliptic. They are definitely not the same size though, and much further apart than is usually drawn.